Alarm light on?

This alarm light is indicating a fault in your system. Normally is illuminates when set after you leave your vehicle. Now it sees a mismatch between what it normally thinks is the "password" in your key.

Eventually the anti theft system will not recognize your key, then the van will only start for 2 seconds then shut off, thinking that it is being stolen.

This guide can help

https://www.2carpros.com/articles/how-to-reset-a-security-system

This is a problem that will have to be addressed by a shop with the correct automotive scanner that can read the values your system is seeing to determine the where fault lies.

Good Luck,

First, please understand there are several sensors that could cause the light to come on. If the light is staying, there is a strong possibility that the security system will eventually not allow the engine to restart for you. It is designed to shut down the ignition so the vehicle can't be stolen. I hate to say it but you need to have the system scanned to identify where the problem is coming from. For me to say what the problem is without scanning the system would be nothing more than a guess. To take it one step further, (and I hate to say it), it needs to be scanned by a dealer. They are the only ones that have the proper test equipment for your vehicle.

Hi lewwms,

Here is a description of the operations and from the symptom description, an error has been detected in the system and appropriate scan tools would be required to perform diagnostics.

OPERATION
The Mechanical Instrument Cluster (MIC) performs a four second bulb check via PCI communication with the Sentry Key Immobilizer Module (SKIM). After the bulb check, the lamp is controlled according to SKIM messages. Then, the SKIM sends messages to the BCM to operate the light based upon the results of the SKIS self tests. The light may be actuated in two possible ways, flashing or on solid. If the light comes on and stays on solid after a power-up test, this indicates that the SKIM has detected a system malfunction. If the SKIM detects an invalid key when the ignition switch is moved to the ON position, it sends a message on the PCI bus to the BCM, to flash the light. The SKIM can also send a message to flash the light and generate a single audible chime at the same time. These two events occurring simultaneously indicate that the SKIS has been placed into the "Customer Learn" mode. Refer to STANDARD PROCEDURE - TRANSPONDER PROGRAMMING for more information on the "Customer Learn" mode. If the light comes on and stays on after the power-up test, diagnosis of the SKIS should be performed using a DRBIII(R) scan tool and the appropriate Body Diagnostic Procedures. The light
is not a serviceable component.

SKIS is the system name.

SENTRY KEY IMMOBILIZER SYSTEM
The Sentry Key Immobilizer System (SKIS) is available as a factory-installed option on this vehicle. It is designed to provide passive protection against unauthorized vehicle use by disabling the engine, after two (2) seconds of running, whenever an invalid key is used to start the vehicle. The SKIS is active whenever the ignition is on and does not require any customer intervention. The primary components of the system are the Sentry Key Immobilizer Module (SKIM), Sentry Key (ignition key with a transponder molded into the head), indicator light, Body Control Module (BCM), and the Powertrain Control Module (PCM). The SKIM is mounted to the steering column with the molded, integral antenna mounted on the ignition housing. The indicator light, is located in the Mechanical Instrument Cluster (MIC).

Your description of the light and neither key working sounds like there is a failure in the system parts. You say it was turning on and then off but not starting then stopped doing that? That is what happens after trying a few times, as you said it locks you out.
Te easiest way to test the system is to use a scan tool that can access the SKIM. It will have indicators to tell if it is the module or elsewhere. For instance it will tell you if it reads the key, passes the signal to the BCM, triggers the start relay or which one isn't working.
There isn't another easy method to test and replacing any parts will require the same level scan tool to program in the replacement parts that you could use to test with.